Rebalance partition_id for ci_builds

Ref: #387301 (closed)

What does this MR do and why?

Following up !109981 (merged)

This MR fixes invalid partition_id for ci_builds on gitlab.com.

It updates all invalid records in production and sets the partition_id to 100.

Database

I choose a default batch size of 5_000 and a sub batch size of 500 as suggested by @krasio as we have 1.5M records to fix.

📝 This seems to be the query executed by our background migration and its associated query plan.

UPDATE ci_builds
SET partition_id = 100
WHERE ci_builds.id BETWEEN 100000 AND 100500
AND ci_builds.partition_id = 101

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